Objective: We aimed to determine the relationship between fasting serum glucose (FSG) concentration and glycated hemoglobin-A1c (HbA1c) in the 3 ethnicities in Singapore after adjustment for demographic and therapeutic variables.Methods: Fasting serum glucose (FSG), HbA1c, and serum creatinine levels were simultaneously sampled from 575 patients with diabetes (389 Chinese, 97 Indians, 89 Malays) in this cross-sectional study between January and May 2008, and the results were subjected to multivariate linear regression analysis.Results: We found a significant interaction between FSG and ethnicity on HbA1c. The correlation between FSG and HbA1c among Chinese subjects was 0.25 (95% confidence interval [CI]:0.2-0.3) relative to the Malays (0.38, 95% CI: 0.30-0.45) after adjustment for age; gender; serum creatinine concentrations; body mass index (BMI); duration of diabetes; use of sulfonylureas, metformin, and insulin; and hemoglobin (Hb) and red cell indices (P = .005). Hence, for a given FSG, the predicted HbA1c will be higher in Malays compared to Chinese subjects. We did not observe a statistically significant difference between Indians and Malays with respect to the correlation between FSG and HbA1c.Conclusion We showed a higher correlation between HbA1c and FSG in Malay subjects relative to the Chinese in this cohort. The ethnic variation in the HbA1c-FSG relationship may be related to differences in percentage contribution by the FSG to overall HbA1c among ethnic groups. Future studies using continuous glucose monitoring (CGM) to elucidate the relative contributions by FSG and postprandial glucose (PPG) to the daily blood glucose profile and the overall HbA1c by ethnicity are required.